It is frequently useful to monitor the density of a liquid used in industrial processes. For instance, during drilling operations of a well bore, drilling mud is circulated through the drill pipe and the annulus of the well bore. The drilling mud serves several purposes in the drilling process, including lubrication and cooling of the drill bit and the removal of cuttings resulting from the penetration of the subsurface formations. Additionally, the weight of the drilling mud in the well bore exerts hydrostatic pressure on the subsurface formations that acts to contain the subsurface pressure encountered during drilling operations. These subsurface pressures may be extremely high, such as 8,000-10,000 psi, or the equivalent of two (2) pounds per square inch per foot of depth. If the hydrostatic pressure of the drilling mud at the depth at which the subsurface pressure is encountered is not equal to or greater than the subsurface pressure, the reservoir fluid will emerge from the subsurface formation and travel to the surface through the well bore. This is known as a blow out and creates extremely dangerous conditions as the pressurized formation fluid generally containing gas rapidly expands as it approaches the surface and atmospheric pressure. However, if the drilling mud hydrostatic pressure is too high, the subsurface formations may be fractured resulting in extensive loss of the relatively expensive drilling mud into the formations surrounding the well bore. Thus, the density of the drilling must must be monitored and adjusted if necessary, to maintain it within a desired range during drilling operations. Another constraint on the density of the drilling mud is that an increase in the density tends to reduce the penetration of the drill bit.
Drilling mud is a liquid composition, usually utilizing oil or water as a base to which various substances are added depending on the properties desired in the drilling operations, the subsurface and environmental conditions, as well as other factors as are known in the art. Typically, the additives include solid particulates or clays to promote the formation of a filter cake on the walls of the well bore and to assist in suspending the cuttings until the drilling mud containing the cuttings reaches the surface, and weighting material ssuch as barites, to control the density of the drilling mud. After circulation through the well bore, the drilling mud is filtered to remove the cuttings carried from the bottom of the well bore and then stored in a mud pit for subsequent use.
Under conventional drilling techniques, a small sample of the drilling mud is periodically obtained as it is deposited in the mud pit. The density of the sample is measured and a determination made whether or not to adjust the density of the drilling mud being used in the well bore. Existing apparatus used to measure the density of the drilling mud sample is called a mud balance and consists of a container having a known volume on one end and a scale and weight on the other end. The apparatus (i.e. mud container, scale and weight) is then placed on a knife edge fulcrum. The container is then filled to its capacity with the sample of interest. The weight is moved along the scale until balanced. Indicia on the scale at the location of the weight indicate the density of the mud sample. That is, the observed weight of the sample may be used to derive a measure of density by dividing the weight by the known volume of the container. The same scale can be calibrated in any set of units desired.
However, this and other conventional density monitoring procedures exhibit certain limitations. For instance, the monitoring of the drilling mud density is performed on an intermittent basis, and must be manually performed. This manual operation takes approximately 10-15 minutes. When conditions are encountered that cause rapid changes in the mud density, the density may drastically change in a matter of minutes or seconds. Frequently, it is not economical or practical to weigh samples of the drilling mud on an ongoing basis, since a skilled technician or engineer is required to perform the test. Less frequent samples introduce a time lag into the monitoring process that is undesirable. Further, manual weighing of a drilling mud sample on a mud balance is relatively inaccurate and requires visual interpretation, thereby introducing the possibility of human error. Finally, weighing of the drilling mud sample must be performed on site and cannot conveniently be conducted remotely. None of the existing methods or apparatus for monitoring drilling mud density adequatley solve the problems outlined above.